Heat or mass transfers between fluid and exchange surfaces are hindered by the presence of boundary layers. Boundary layers refer to regions where fluid flow near walls of the exchange surface is relatively stationery compared to the bulk of the fluid away from the walls. Flow promoters are structures that can be inserted at the walls of the exchange surface to disrupt the boundary layers, thereby facilitating heat or mass transfer.
In membrane separation processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), electrodialysis, membrane distillation etc., operational issues such as temperature polarisation, concentration polarisation and fouling are consequences of poor hydrodynamic conditions due to boundary layers. For example, concentration polarisation happens when solutes rejected by a membrane accumulate near the membrane surface causing high solute concentration near the membrane surface, greater than the bulk concentration. High degree of concentration polarisation can exacerbate fouling. Therefore, it is important for membrane modules to have good fluid management in order to reduce the effect of concentration polarization and fouling.
Conventional plastic net-typed spacers, typically produced via common extrusion methods and ranging between 0.6-0.9 mm in thickness are used to improve the heat or mass transfer, by static mixing or creating eddies. A conventional mesh spacer generally has two layers of filaments. The filaments in the same layer are parallel to each other, and they cross the filaments in the adjacent layer at an angle. The conventional method of making spacers or flow promoters for membrane modules involved the extrusion of polypropylene (PP) materials into two layers of filaments. The first layer of filaments was placed parallel to each other at regular intervals specifically known as the mesh length. The second layer of filaments was welded at an angle known as the hydrodynamic angle on top of the first layer. Other methods described in previous patent and literature include vacuum foaming, embossing, fusion bonding, pressing with a die, milling and 3D printing techniques such as SLS, FDM, SLA and Polyjet.
The geometrical characteristics of conventional spacers on mass transfer and pressure drop have been extensively studied [3-7]. However, conventional spacers are associated with several issues such as pressure loss, fouling, and maldistribution of flow. For example, the presence of a spacer in the flow channel hinders fluid flow, causing a drop in pressure across the membrane module, while fouling problems are found to be associated with the accumulation of foulants on the spacer as the spacer may provide localized dead spots with poor mass transfer that initiate fouling. As more knowledge about conventional spacers was obtained, researchers began to design novel spacers with modified structures for further enhancing filtration performance.
Prior work on spacer modifications was aimed at altering and optimizing the geometry of the conventional spacer filaments. A variety of shapes has been proposed for modifying the cross-section of the filaments [8, 9], which usually has a cylindrical cross-section. The work on altering the filament diameter, angles, and other geometrical properties can be also found in the published papers and patents [10, 11]. A special spacer design was reported that replaced the cylindrical filaments with twisted tapes [12, 13]. Several novel designs were proposed to optimize the shape of the spacer filaments [8, 9], the topological structures of the network [10, 11], and the curvature of the fluid channels formed by the spacer filaments [12, 13, 19].
In addition to the modifications of the filaments, some studies focused on optimizing the structures of the network by replacing the single layer network with a multi-layer structure that has different geometric characteristics for each layer [14-16]. For example, Schwinge et al. [14] added an additional layer of filaments into the conventional spacer mesh that consists of filaments crossing each other in two directions. The idea of this design is to reduce the void space in the fluid channel without increasing the membrane area covered by the spacer. The spacers proposed by the University of Twente [15, 16] adopted a network that sandwiches a spacer with normal or modified filaments between two thinner spacers, which contact the membrane surface in a more intimate manner. Although these spacer designs are able to markedly enhance the mass transfer, they inevitably suffer from the increase in the pressure drop through the membrane module, thereby resulting in an increase in the energy consumption.
Some designs were proposed in an attempt to balance these opposing effects. For example, zigzag and sinusoidal spacers were investigated by computational fluid dynamics [17-19]. Xie et al. [19] recently employed sinusoidally shaped spacer filaments for enhancing the performance in reverse osmosis (RO) processes. The smooth tortuous channels could enhance the extent of turbulence and were expected to reduce the hydrodynamic resistance while avoiding a significant increase of the hydraulic resistance. However, the study indicated a sharp increase in the pressure drop when the amplitude and spatial frequency of the sinusoidal channel were increased to certain critical values and its wavelength was shortened to some critical values. Moreover, these tortuous structures increased the propensity of fouling.
Research on optimisation of spacers mainly involved experiments on modification of commercial feed spacers. While there are limited works on innovative spacer geometries [14-16, 19, 28-38], some were solely simulated via CFD to study the effect of geometrical designs on spacer performance in order to achieve an optimal spacer design with good performance [8, 39-41]. However, difficulties in manufacturing complex spacer designs made it not possible to validate the CFD simulations. To date, there has been limited experimental work reported to confirm the direct impact of spacer geometry on fouling.
It should be noted that the boundary layer mass coefficients (k) provided by current spacers limit the potential of next generation high permeability membranes. This is because concentration polarization is related to the ratio (flux/k), so higher flux requires higher k; novel spacers may be able to provide higher k without high pressure losses. The second issue of membrane fouling is a process whereby a thin film of foulant is deposited on a membrane surface so that the overall performance of membrane is decreased. Membrane fouling can cause severe flux decline and hence require higher transmembrane pressure to maintain the same flux. Severe membrane fouling may require intense chemical cleaning or membrane replacement, which further increases the operating costs of a desalination plant.
A spiral wound module (SWM) is the workhorse for RO processes. It has complex hydrodynamics due to the presence of the feed spacers. Membrane fouling of a SWM is further complicated by the permeation of water through the membrane while the solute is rejected by the active layer of the membrane and hence concentration polarization on the membrane surface and pressure loss along the channel. The filtration performance of a SWM can be significantly affected by the spacers. They not only define the space between the membrane leaves, but also play a role as a turbulence promoter (some studies refer to them as an eddy promoter since the flow may not be fully developed [1, 2]). Schwinge et al. indicate that the spacer in a fluid channel can intensify the mass transfer, thereby mitigating the negative effects caused by the concentration polarization (CP) and membrane fouling [2].
It is therefore of primary importance to optimize spacers for better filtration performance. This is a key to the exploitation of next generation high flux RO membranes because it is necessary to maintain the ratio of flux to boundary layer transfer coefficient to avoid excessive CP and fouling.